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Will a Dirty Filter Affect my Air Conditioning Unit?

In short, yes. The filter in your AC unit is vital for keeping dust and debris away from the moving parts of the machine, but these substances can build up over time, causing a variety of problems.

These issues can often be avoided entirely by changing the filter approximately every 90 days, or more often if the building is damp or used by someone susceptible to allergies. If you hold a maintenance contract for your air conditioning units, the filter can also be replaced as part of a regular check and repair.

When you believe your HVAC filter might be dirty in-between checks, it is important to replace it as soon as possible for the sake of your electricity bills, the life span of your unit and the health of the building’s occupants.

Have you got any questions about air conditioning maintenance? Call us on 01179 523355 or fill in our contact form with any queries you may have and we will get back to you.

Nine Problems caused by a dirty AC filter

  1. Uneven Heating and Cooling

A dirty HVAC filter can impede the proper airflow which is essential for the consistent heating and cooling of your property. When a filter is dirty or the machine otherwise needs maintenance, you may notice significantly cooler and warmer areas in different parts of the building. This in turn can affect the comfort of those who reside in or use the property.

  1. Freezing

A clean air filter will allow air to effortlessly flow in and out of the device. If the filter is clogged with dirt, however, cold air can become trapped inside the unit. In more extreme cases, this can result in internal freezing- where ice crystals form in delicate parts of the machinery. Over time, this issue can cause serious damage to the air conditioner and you may end up paying for extensive repairs to your unit.

  1. Mould Growth

This is particularly a problem in humid environments but can occur anywhere. Water vapour is naturally created in AC ducts when cold air passes through but in most cases, it evaporates. Where it is allowed to condense and collect, however, mould spores will thrive in the moist environment and feed off the dust your blocked filter was unable to keep from building up in the ducts.  This is particularly true if the dirty filter is also preventing the system from cooling properly, as mould doesn’t grow in very low temperatures. Mould in the system is a health risk to anyone using the building and must be dealt with swiftly.

  1. Clogged Blower

As the filter collects airborne material, it will slowly begin to prevent the blower from working at full capacity. If the filter is especially dirty, it can impede the blower from expelling air altogether. This can cause the machine to shut down.

  1. Short Cycling

AC units are meant to cycle on and off a few times throughout the day. You may be able to recognise cycles by the louder noise they produce and they are also sometimes accompanied by a temperature change. When a unit is installed with an air filter which had become dirty, these cycles can become much more frequent. Not only does this waste energy, but the associated sounds and sensations can become an irritant to anyone using the building.

  1. Worse indoor air quality

Filtered air is a benefit of investing in an air conditioning unit. When a unit is working correctly, it should filter out many contaminants and allergens, giving you better quality indoor air. This benefit is compromised if your filter is dirty as the system will no longer be as effective at keeping contaminants at bay.

woman ill and sneezing due to dirty air conditioning filters

  1. Allergen Build-up

It’s not only water and mould that can build up inside your ducts. When your filter is dirty, allergens such as pollen, dust and pet dander can linger there too. When you run a forced air system, all those allergens will be released into the air, which can be particularly uncomfortable for those who suffer from asthma or other allergies. The pollutants which your air filter collected when it was working properly can also be blown back out into the air system.

  1. Higher Energy Use

One of the reasons someone may choose to replace their filter less regularly than recommended is due to the cost. This decision can, unfortunately, cause you to lose money in other ways. When your unit has a saturated filter, not only will it work less effectively but it will have to use much more power to achieve this sub-standard result. This can mean paying high utility bills which easily could have been avoided.

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  1. Shorter Unit Lifespan

A clean, good quality filter will extend the life of your Air Conditioning system by keeping its inner workings free of debris. When you are using a filter which is unable to perform this function well, you can expect the machine to need repairs more frequently. It is also likely to be a shorter amount of time before a replacement is needed.

technician soaking ac filters

What does a Dirty Air Filter Look like?

Especially if your unit has been suffering from one or more problems mentioned above, you may be able to guess your filter needs replacing without further investigation. There are, however, a few more warning signs to look out for.

One of the most noticeable signs is the area around the AC vents becoming dustier than the rest of the property. If you’re unsure whether you’re imagining this or not, you can try something which is known as “The White Sheet Test.” This involves hanging a white sheet or pillowcase approximately 5 inches away from a vent and leaving it for an hour. At the end of that time, you will be able to judge the cleanliness of your air filter by the colour of the sheet. The grayer the sheet, the dirtier the filter.

Another clue that the filter may need to be replaced can be found by placing your hand on the back of the unit. If it feels hotter than usual or you can feel warm air being expelled, it may show that the internal airflow has been impeded or the unit is having to use more energy than normal to make up for its dirty filter.

The state of your filter should also be obvious when you remove it. Most filters take on a brown or greyish hue when they are dirty and you may be able to see dust clinging to the surface.

technicial putting back air conditioner filter after cleaning

Can I run my AC without a filter?

Technically yes, but we don’t recommend that you try it. HVAC filters are a very important part of your system and neglecting them can cause a range of unpleasant and expensive problems.

Without a filter for the dust and dirt, these substances will continually be pumped through the unit and the air in your home without settling. This is likely to end in one of two scenarios: someone breathing the dust in while it’s airborne or it sticking to the moist tubes inside your AC unit. The first instance will be uncomfortable for whoever occupies the building, the second is likely to cause problems like machine breakdown or mould in the AC ducts.

Dirt is the top reason for air conditioner breakdowns, so it is wise to protect the internal system from it to the best of your ability.

ac technician vacuuming air conditioner filters


Filters are vital in keeping your AC unit in top working order, but a dirty filter can be almost as bad as no filter at all. Make sure to change the filter regularly to give your unit the best possible chance of a long life working at peak efficiency.

Filters are usually simple to remove and replace yourself but, a few times a year, you may want this done as part of your scheduled Air Conditioner maintenance, carried out by a professional.

If so, our experienced air conditioning engineers have the expertise you need.

The TEK air conditioning maintenance division covers Bristol, Bath, Gloucester, Cheltenham, Swindon, and surrounding areas, allowing us to provide a quick maintenance service to our local clients.

Even if you didn’t have your system installed by us, we can still offer a Managed Maintenance Contract to new clients throughout the South West.